Phosphoglucose isomerase

Phosphoglucose Isomerase is a multifunctional protein mainly implicated in playing a role as a housekeeping enzyme in the glycolysis pathway. Outside the cell, it is known to assume different functions including - Hence it commonly referred to as moonlighting protein.
 * Neuroleukin ( nerve growth factor) - Neuroleukin is secreted by T cells and promotes the survival of some embryonic spinal neurons and sensory nerves. It also causes B cells to mature into antibody-secreting cells
 * Autocrine motility factor - AMF is a product of tumor cells that stimulates cancer cell migration and may be involved in cancer metastasis and invasion
 * Maturation and differentiation (mediator)/factor- DMM was isolated from T cell culture media and shown to cause in vitro differentiation of human myeloid leukemia HL-60 cells to terminal monocytic cells.

Structure
PGI is active as a dimer with a subunit molecular mass of approximately 55 kDa ( ~555 aa in Rabbit) - two A/B sandwich domains in each subunit.

Mechanism
PGI interconverts Glucose-6-Phosphate and Fructose-6-Phosphate in both the glycolysis and gluconeogenesis pathways. The proposed mechanism for sugar isomerization involves several steps and is thought to occur via general acid/base catalysis. Since glucose 6-phosphate and fructose 6-phosphate exist predominantly in their cyclic forms, PGI is believed to catalyze first the opening of the hexose ring to yield the straight chain form of the substrates. Glucose 6-phosphate and fructose 6-phosphate then undergo isomerization via formation of a cis-enediol intermediate with the double bond located between C-1 and C-2. This mechanism contains features of the mechanisms for two other sugar isomerases, triose phosphate isomerase, which uses a proton-transfer mechanism via a cis-enediol intermediate.

Additional Resources
For additional information, see: Carbohydrate Metabolism